A conventionally used gas sensor detects an oxygen concentration included in the exhaust gases which are exhausted from an internal combustion engine or the like for air-fuel ratio control. For example, a gas sensor where a gas sensor element, which includes a so-called cup-shaped solid electrolyte, a reference electrode 220, a measuring electrode and a heater, is incorporated (refer to Patent Document 1, for example). The reference electrode 220 is disposed on an internal surface of the solid electrolyte. The measuring electrode is disposed on a lateral surface on the gas sensor. The heater is disposed on an inside surface of the solid electrolyte. The gas sensor is used in a state that the solid electrolyte is heated and activated by the heater.